1. Field of the Invention
The present invention relates to a cationic paint composition and more specifically relates to a cationic electrodeposition paint composition capable of forming a cured coating film excellent in performances such as a throwing power, a rust preventive steel plate aptitude, etc.
2. Description of the Prior Art
A cationic electrodeposition paint composition has been used mainly for undercoating of a car body as an electrodeposition paint and other wide range of applications, and products having various properties have been developed up to now. For example, there has been proposed a cationic electrodeposition paint composition which comprises a base resin containing an epoxy resin which comprises amino group and/or quaternary ammonium group as a water-solubilizing group and is internally plasticized with a plasticizer such as polyamide, polyester, polyether, etc., and which is excellent in corrosion resistance and good in rust preventive steel plate aptitude and adhesion. In such a cationic electrodeposition paint composition, there are compounded lead compounds or chromium compounds, for example, lead chromate, basic lead silicate, strontium chromate, etc. as an anticorrosive pigment. Recently, however, the use of such harmful compounds such as lead compounds or chromium compounds is restricted in terms of pollution problems, and the development of a cationic paint composition, excellent in corrosion resistance without compounding lead compounds or chromium compounds, has been strongly desired.
As an epoxy resin which is internally plasticized with a plasticizer tends to deteriorate the corrosion resistance of the coating film, it may be considered to improve the corrosion resistance by using an epoxy resin without containing a plasticizing modifier as a base resin. There is, however, a problem that a paint composition using a non-plasticized epoxy resin has a lower rust preventive steel plate aptitude.
Moreover, it has been proposed to compound to an epoxy resin a plasticizer which does not deteriorate the corrosion resistance of the coating film, for example, polyol resins such as polyester polyol, polyether-polyol, polyurethane polyol, acrylpolyol, etc.; polyolefin such as polybutadiene, polyethylene, etc. There are problems, however, that these materials are not compatible enough with an epoxy resin and are not only poorly effective to improve the rust preventive steel plate aptitude but also deteriorate the corrosion resistance of the coating film if compounded in a large amount.
On the other hand, a throwing power is required to a cationic electrodeposition paint composition as a rust preventive countermeasure for baggy parts of a car body (locker, side-sill, and pillar) etc. Particularly, a higher throwing power has been recently required to a cationic electrodeposition paint composition because of a more complicated body structure, such as putting reinforcement to the locker part and in the side-sill for levelling up of a car safety level. Therefore, it has been desired to develop a cationic electrodeposition paint composition having a high throwing power, and furthermore, excellent in corrosion resistance and rust preventive steel plate aptitude.